Device for pressure monitoring

ABSTRACT

A device for monitoring pressure within a gas space, which is filled with gas, preferably nitrogen, and/or is prestressed, of a pressure accumulator of a hydraulically driven percussive mechanism having a housing in which a piston is mounted, together with a display element which extends through the face surface of the housing, so as to be slidingly movable counter to the force of a spring, wherein the piston divides the housing into a pressure chamber and a spring chamber. The spring acts on a spring chamber-side working surface of the piston, and the pressure accumulator is at least indirectly operatively connected to the pressure chamber-side working surface of the piston, such that, in the event of a critical pressure within the pressure accumulator being undershot, the piston and the display element are displaced to such an extent that the display element protrudes out of the housing.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119 ofGerman patent application DE 10 2014 108 848.4 filed Jun. 25, 2014, theentire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a device for monitoring the pressurewithin a gas space, which is filled with a gas, preferably nitrogen,and/or is prestressed, of a pressure accumulator of a hydraulicallydriven percussive mechanism, in particular of a demolition hammer or ofa drilling hammer, having a housing in which a piston is mounted,together with a display element which extends through the face surfaceof the housing, so as to be slidingly movable counter to the force of aspring, wherein the piston divides the housing into a pressure chamberand a spring chamber.

BACKGROUND OF THE INVENTION

Hydraulically operated percussive mechanisms are used in mountedimplements, such as for example hydraulic hammers or drilling hammers,wherein the mounted implements are mounted on carrier vehicles, inparticular mobile excavators, and are connected to the hydraulic systemthereof via a pressure line and a return line. Percussive mechanismshave a percussive piston which has one or more hydraulic drive surfaces,at least one of which is, by way of a valve, connected alternately to areturn line, which is at low pressure, to the tank of the carriervehicle or via a pressure line, which is at high pressure, to the pumpof the mounted implement, such that the percussive piston performs anoscillating movement along its longitudinal axis. During normaloperation, at the end of its movement in one movement direction, thepercussive piston strikes a tool, wherein the tool is a chisel, anadapter for piledriving or pipe driving, or an anvil arranged betweenthe percussive piston and the tool.

Hydraulically operated percussive mechanisms have, in some cases, apressure accumulator in the form of a piston accumulator in order tostore kinetic energy of the percussive piston. The upper, cylindricalend, situated opposite the tool, of the percussive piston projects intoa gas-filled gas space of the pressure accumulator, wherein a seal whichbears against the end of the piston prevents an escape of the gas alongthe percussive piston.

As the piston moves in the direction of the gas space during the returnstroke, the end of the piston displaces gas within the gas space, whichthus decreases in size, leading to an increase in the gas pressure. Thecompressed gas exerts a force on the end of the piston, said forceincreasing as the gas volume decreases in size. Said force is utilizedto accelerate the piston in the direction of movement toward the tool.

During operation, there are thus three characteristic piston positionswhich can be associated with a respective gas pressure. For example, ifa hydraulic hammer which has a percussive mechanism is raised or setdown horizontally, its percussive piston is situated in the lower restposition, in which the gas pressure in the piston accumulator assumesits lowest value. When the processing of a piece of material using ahydraulic hammer is ended and the operation of the percussive mechanismis stopped, in order to position the hydraulic hammer differently, thepercussive piston assumes its rest position. If the chisel is pressedwith its tip against material, the chisel is pushed into the housing ofthe percussive mechanism until it comes to rest against a stop. In thiscase, the percussive mechanism is pushed in the upward return strokedirection, in the direction of the gas space, and assumes the impactposition, wherein the gas pressure in the piston accumulator assumes avalue higher than that in the rest position. When the percussivemechanism is activated, the percussive piston is then hydraulicallymoved further in the return stroke direction until it reaches its upperreversal point, at which the gas pressure assumes its highest value,wherein the position at the upper reversal point is dependent on theusage conditions of the percussive mechanism and the operating pressureand the pressure in the piston accumulator, and may therefore vary.

Owing to leakage along the seals and gas flows through the seal or thediaphragm or bubble diffusion, the gas pressure falls over the course oftime. To maintain the effectiveness of the accumulator, it is thusnecessary for the fill pressure in the accumulator to be checked atregular intervals and for the accumulator to be replenished with gas ifrequired. For checking the gas pressure, fittings are necessary for theconnection of a pressure measurement implement, for example a manometer,to the gas space. Such fittings comprise a hose and a manometer and, forfilling and release purposes, also discharge and filling valves, apressure reduction valve and screw connections in order for the fittingsto be connected to a gas storage bottle. On the gas space, there isprovided a shut-off valve or a mechanically opening check valve, towhich the fittings are connected, and in addition, normally also asealing closure screw for preventing an undesired escape of gas.

Since, during the operation of the percussive mechanism, the gaspressure in the piston accumulator constantly changes in a mannerdependent on the position of the percussive piston, the gas pressure inthe piston accumulator must be measured when the percussive piston is ina particular and defined position, which is possible only when thepercussive mechanism is deactivated, that is to say when the hydraulicsystem is unpressurized. To measure the gas pressure, the rest positionof the percussive piston is used, as the percussive piston is situatedin a geometrically defined position, specifically at the lower stop.

The checking of the gas pressure is time-consuming because, before themeasurement, components such as closure screws must be released, screwconnections must be made and, after the measurement, the connection mustbe released again and the closure screw screwed in again.

Failure to carry out regular checks of the gas pressure can result in adrop in the gas pressure, which reduces the effectiveness of theaccumulator and impairs the performance of the percussive mechanism and,for example as a result of excessively intense pressure fluctuations,can also lead to damage of components. The fittings and equipment forthe checking of the gas pressure must be available for the checking ofthe gas pressure and must be operational, and the user must be familiarwith the use of the fittings.

JP 2008114296 has disclosed a gas pressure indicator device which isarranged on the hydraulic chamber and in the case of which the gaspressure of the pressure accumulator acts directly on a spring-loadedpiston which, with increasing gas pressure, is displaced in thedirection of the spring. If the piston is displaced, a bar which isconnected fixedly in terms of motion to the piston protrudes out of thehousing of the indicator and reveals one or more marks, for example inthe form of a groove. The higher the gas pressure acting on the piston,the further the piston is displaced, and the further the bar protrudesout of the housing. During the filling of the accumulator, the markingindicates a particular gas pressure. After measurement has beenperformed, by virtue of a cap being screwed onto the housing of thedevice, the bar and the piston are pushed in again counter to thepressure of the gas, in order that the piston and the bar do notpermanently move owing to the fluctuating gas pressure during operation.To activate the indicator device, the cap must be removed again.

Percussive mechanisms may furthermore have pressure accumulators in theform of a hydraulic accumulator for the purpose of storing pressurizedoil of the hydraulic system. The gas space of a hydraulic accumulator isseparated from an oil space by way of a separating element in the formof a piston, an elastic diaphragm or an elastic, hose-like or pot-shapedbladder. The oil space is connected to the hydraulic system directly orvia a throttle or a valve. In order to check the gas pressure or forfilling purposes or in order to release the gas charge, correspondingfittings such as manometers, hoses and pressurized gas bottles can beconnected by way of a valve which is connected to the accumulator space.If, in the hydraulic system, an operating pressure prevails which ishigher than the fill pressure within the gas space, oil flows into theoil space and displaces the separating element in the direction of thegas space, whereby the oil space is increased in size, the gas space isreduced in size and the gas is compressed, to a pressure correspondingto the pressure of the oil. Thus, when the oil demand of the consumer islow, the oil that is delivered by the pump can be stored in thehydraulic accumulator in order to reduce the pressure increase in thehydraulic system, and when the demand of the consumer is high, oil canbe released from said hydraulic accumulator in order to reduce thepressure drop in the hydraulic system. Intense pressure fluctuations andpressure peaks are thus avoided.

In the case of known indicator devices, it has proven to bedisadvantageous that the indicator device must firstly be activated by auser by the removal of the cap, such that there is the risk that theuser, in failing to adhere to the service intervals, neglects to performsuch activation and the percussive mechanism is operated with anexcessively low gas fill pressure, whereby the performance of thepercussive mechanism is reduced and components are possibly damaged.

Even if the indicator device were always activated by virtue of the capbeing left off, and wear of the indicator device were accepted, it wouldbe the case even in the presence of an excessively low gas fill pressurethat, during operation, the gas pressure would repeatedly exceed therequired gas fill pressure, and the bar would repeatedly emerge from thehousing such that the marking or markings would appear, which marking ormarkings would then at least intermittently indicate an adequate gasfill pressure. If this signal is interpreted incorrectly, anundershooting of the required gas fill pressure will not be noticed, andthe percussive mechanism will consequently be operated with anexcessively low gas fill pressure, resulting in a reduction in theperformance of the percussive mechanism and the risk of damage tocomponents.

Since the length to which the bar protrudes beyond the housing isdependent on the gas pressure and increases with increasing gas pressureup to a maximum structurally limited length, the user must measure thelength or take notice of whether one or more markings present on the barare visible. The reading of the indicator is thus cumbersome andrequires knowledge regarding the interpretation of the length or of themarkings.

An undershooting of the required fill pressure is not clearly indicated,because during operation of the percussive mechanism, when thepercussive piston is situated above the rest position, an adequate gasfill pressure is indicated.

SUMMARY OF THE INVENTION

It is therefore the object of the present invention to propose a devicefor monitoring the pressure, which device clearly indicates anundershooting of the required gas pressure in a pressure accumulatorboth while the percussive mechanism is at a standstill and while it isin operation, without the need to perform handling in order to activatethe device. Here, the device should output a signal only when thepercussive piston is situated in its defined rest position, in which thegas pressure is measured.

The object is achieved by means of the device of the present invention.According to the invention, the spring acts on a spring chamber-sideworking surface, which is averted from the display element, of thepiston, and the pressure accumulator is at least indirectly operativelyconnected to the pressure chamber-side working surface, facing towardthe display element, of the piston, such that, in the event of acritical pressure within the pressure accumulator being undershot, thepiston and the display element are displaced to such an extent that thedisplay element protrudes out of the housing. By contrast, in thepresence of the desired pressure conditions, the pressure within thepressure chamber of the housing is of such a magnitude that the springforce is not sufficient to push the indicator element out of thehousing, such that the piston remains in a rest position regardless ofthe pressure within the pressure chamber, and the indicator element doesnot indicate an undershooting of the critical pressure.

In other words, the gas pressure acting in the gas space of the pressureaccumulator acts on a first surface of the piston of the indicatordevice (pressure chamber-side working surface), wherein the firstsurface is arranged on that side of the piston to which a bar, as apreferred indicator element, is also fastened, such that the gaspressure exerts a force on the piston in the direction in which the baris retracted into the housing. A spring is arranged on the secondsurface, which is directed oppositely to the first surface, and impartsa force counter to the gas force. The spring and the piston surface aredesigned such that, in the presence of an adequate gas fill pressure,the gas force exceeds the spring force and the piston and the bar assumea first, structurally defined position in which the bar is fullyretracted into the housing of the indicator device. The gas pressure atwhich the indicator is intended to output a signal is defined by way ofthe preload of the spring. If the gas pressure falls below thepredefined gas fill pressure defined by the spring force, the piston isdisplaced by the force of the spring, such that the bar protrudes out ofthe housing and appears to the user of the percussive device. The springmay for example be a compression spring with coils composed of steelwire, or may be a gas pressure spring which, by way of a gas preload,acts on a second surface situated opposite the first surface. Thefurther the gas pressure falls, the further the bar protrudes out of thehousing, until it is stopped by a structurally defined stop.

A device of said type advantageously outputs a signal only when the gaspressure falls below a preset threshold value. This is particularlyimportant because, even if the gas fill pressure was previouslycorrectly set, the gas pressure can fall over the course of time owingto leakages, leading to a reduction in the performance of the percussivemechanism and possibly to damage of percussive mechanism components.Even in the case of a percussive mechanism which is at leastintermittently shut down, it is possible from the protrusion of theindicator element to directly identify that the gas fill pressure hasfallen below its required value.

Since it is the case that the gas pressure is higher in all positions ofthe percussive piston, with the exception of the rest position, in whichthe percussive piston assumes a structurally specified defined positionand in which the gas fill pressure is to be measured, than in the restposition, it is the case that, when the required gas fill pressureprevails, a movement of the piston and of the rod during the operationof the percussive mechanism is prevented, thus considerably increasingthe service life of the components of the gas pressure indicator.

It is thus always the case that, when the signal appears, the requiredfill pressure in the pressure accumulator has been undershot, andreplenishment of gas is necessary.

The device according to the invention can also be used on hydraulicaccumulators.

Preferred refinements of the present invention will be described below.

In a first preferred refinement, it is provided that the display elementhas at least one marking which, after the undershooting of a criticalpressure and the displacement of the display element, appears andsignals the undershooting of the critical pressure. It is also possiblefor multiple axially spaced-apart markings to be provided, theappearance of which represents a measure for the value of the pressure.

In a further preferred embodiment of the present invention, theindicator element is a bar which is connected to the pressurechamber-side working surface of the piston. When the required pressurewithin the pressure chamber of the housing undershoots a predefinedvalue, the bar protrudes out of the housing owing to the spring force,and thus signals an undershooting of the predefined pressure.

Furthermore, the pressure accumulator is preferably connected directly,via a pressure line, to the pressure chamber-side working surface of thepiston. Such an embodiment can be produced at relatively low cost andhas the effect that an undershooting of the differential pressurebetween the pressure accumulator and the spring chamber causes theindicator element to be pushed out of the housing and expose themarking, such that the undershooting of the required pressure issignaled.

Alternatively, in a particularly preferred refinement, it is providedthat the pressure chamber of the pressure accumulator is connected tothe control surface of a spring-loaded pressure valve which can betransferred from a pass-through position into a blocking positioncounter to the force of a compression spring, wherein the pressurechamber of the housing is connected via a check valve to the pressureline of the hydraulics and, in the pass-through position, is relieved ofpressure to the tank. After the initial activation of the hydraulics,the pressure prevailing in the pressure chamber is the same as thatwhich normally prevails in the hydraulic line, which far exceeds thecritical pressure value within the pressure accumulator. In thisrespect, in the case of this setting of the pressure valve, theindicator element is permanently held in the position in which theindicator element is mounted to the greatest possible extent within thehousing. Only when the critical pressure within the pressure accumulatoris undershot does the spring-loaded valve switch into the pass-throughposition, such that the pressure within the pressure chamber flows out,and consequently the low pressure is indicated by the displacedindicator element. In this embodiment, the blocking position may also bereplaced by a check valve which is accordingly integrated within thespring-loaded pressure valve, whereby a hydraulic line is advantageouslydispensed with.

Finally, the check valve is preferably arranged such that a flow ofhydraulic fluid from the pressure line of the hydraulics via the checkvalve to the pressure chamber is possible, but a return flow from thepressure chamber via the check valve is blocked.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific objects attained by its uses,reference is made to the accompanying drawings and descriptive matter inwhich preferred embodiments of the invention are illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic view of a hydraulic percussive mechanism accordingto a first embodiment;

FIG. 2 a is a schematic view of a hydraulic percussive mechanismaccording to a second embodiment;

FIG. 2 b is a schematic view of a hydraulic percussive mechanismaccording to a second embodiment; and

FIG. 3 is a schematic view of a hydraulic percussive mechanism accordingto a third embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hydraulically operated percussive mechanisms 1 are used in mountedimplements such as hydraulic hammers, drilling hammers etc., wherein themounted implements are mounted on carrier vehicles, such as for examplemobile excavators, and are connected to the hydraulic system 2 thereofvia a pressure line 3 and a return line 4. On the carrier vehicle thereis provided a switching valve which can break or establish theconnection between the pump of the carrier vehicle and the pressure portof the percussive mechanism and between the tank of the carrier vehicleand the return port, in order to deactivate or activate the percussivemechanism.

Percussive mechanisms have a percussive piston 5 which has one or morehydraulic drive surfaces 6, 7, at least one of which can, by way of avalve 8 associated with the percussive mechanism, be connectedalternately to a return line, which is at low pressure, to the tank 9 ofthe carrier vehicle or via a pressure line, which is at high pressure,to the pump 10 of the mounted implement, such that the percussive piston5 performs oscillating movements along its longitudinal axis. Duringnormal operation, at the end of its movement in one movement direction,the percussive piston strikes a tool 11, wherein the tool is a chisel,an adapter for piledriving or pipe driving, or an anvil arranged betweenthe percussive piston and the tool.

Hydraulically operated percussive mechanisms have, in some cases, apressure accumulator 12 in the form of a piston accumulator in order tostore kinetic energy of the percussive piston. The upper, cylindricalend 13, situated opposite the tool, of the percussive piston projectsinto a gas-filled gas space 14 of the pressure accumulator, wherein aseal (not illustrated) which bears against the end of the pistonprevents an escape of the gas along the percussive piston.

As the piston moves in the direction of the gas space during the returnstroke, the end of the piston displaces gas within the gas space, whichthus decreases in size, leading to an increase in the gas pressure. Thegas exerts a force on the end of the piston, said force increasing asthe gas volume decreases in size. Said force is utilized to acceleratethe piston 5 in a direction of movement toward the tool 11.

During operation, there are thus three characteristic piston positionswhich can be associated with a respective gas pressure. For example, ifa hydraulic hammer which has a percussive mechanism 1 is raised or setdown horizontally, its percussive piston is situated in the lowermostposition, the rest position, in which the percussive piston bearsagainst a piston stop 16 of the percussive mechanism housing and inwhich the gas pressure in the piston accumulator assumes its lowestvalue. Every time the processing of a piece of material using ahydraulic hammer is ended and the operation of the percussive mechanismis stopped, in order to position the chisel 11 on a different piece ofmaterial 100, the percussive piston assumes said rest position. If thechisel, as illustrated in FIG. 1, is pressed with its tip againstmaterial, the chisel is pushed into the housing of the percussivemechanism until it comes to rest against a stop 15. In this case, thepercussive mechanism is likewise pushed in the upward return strokedirection, in the direction of the gas space, and assumes the impactposition, and the gas pressure in the piston accumulator assumes a valuehigher than that in the rest position. When the percussive mechanism isactivated, the percussive piston 5 is then hydraulically moved furtherin the return stroke direction until it reaches its upper reversalpoint, at which the gas pressure assumes its highest value, wherein theposition at the upper reversal point is dependent on the usageconditions of the percussive mechanism 5 and the operating pressure andthe gas fill pressure in the piston accumulator, and may vary slightly.

The percussive mechanism illustrated in FIG. 1 has a gas pressureindicator device 50, in the case of which the gas pressure, afterreaching or undershooting a particular gas fill pressure, outputs asignal by virtue of a bar 23 being deployed out of a housing andprotruding beyond the housing surface 29.

The gas pressure indicator device 50, which is composed of a piston 22guided in movable fashion in a housing equipped with a bore 21, has abar 23 connected fixedly in terms of motion to the piston and has aspring 24. The piston divides the bore 22 into two chambers, in which ineach case one piston surface 25, 26 is situated. A first piston surface26 (pressure chamber-side working surface) is connected by way of a line27 to the gas space 14 of the pressure accumulator 12, such that the gaspressure prevailing in the gas space acts on the piston surface 26 andexerts on the piston a gas force which can displace the latter towardthe left, in the direction away from the housing surface 29. The spacesurrounding the second piston surface 25 is either connected to theatmosphere or sealingly closed off and filled with air at low pressure,such that the pressure on said surface does not have a significantinfluence on the movement of the piston. A spring 24 bears against the(spring chamber-side) working surface 25 of the piston, and exerts onthe piston a force acting counter to the gas force.

The surface area of the pressure chamber-side working surface 26 and theforce of the spring 24 are configured such that, during the operation ofthe percussive mechanism, and in the presence of an adequate gas fillpressure in the gas space 14, the gas pressure is sufficient to displacethe piston into its left-hand end position and hold it there, in whichposition the spring length is at its shortest and the bar is retractedinto the housing to such an extent that it no longer protrudes beyondthe housing surface 29.

Only when the percussive mechanism is deactivated and the percussivepiston 5 assumes its lowermost rest position, in which the percussivepiston bears against a stop 16 of the percussive mechanism housing andin which the gas pressure is to be measured, does the volume in the gasspace assume its greatest value and the gas pressure assume its lowestvalue within the range of the possible percussive piston movement.

When the percussive piston is situated in the rest position, if the gaspressure is below the predefined target value, the gas force falls belowthe spring force, whereby the piston is displaced to the right, and thebar 23 is likewise displaced and emerges from the housing and protrudesbeyond the housing surface 29. The lower the gas pressure, the furtherthe bar protrudes beyond the housing surface. In the position of thepercussive piston illustrated in FIG. 1, it would be necessary, contraryto the illustration, for the bar to be retracted fully into the housing,as the gas pressure always rises in percussive piston positions abovethe rest position and always lies above the target value when the gasfill pressure is correctly set.

A signal is output to the user, by way of the appearance of the bar,only when the percussive piston is situated in the rest position, inwhich the gas pressure is to be meaningfully measured and the gaspressure has fallen below the required target pressure.

The risk of the percussive mechanism being operated with an excessivelylow gas pressure in the accumulator, which can lead to performancelosses and damage to the components of the percussive mechanism, isgreatly reduced by way of this embodiment, as the signal of theprotruding bar is clearly visible to the user, that is to say if the barappears, the gas fill pressure has fallen below the target value and thegas space must be replenished with gas.

The embodiment of the gas pressure indicator device 70 for a percussivemechanism illustrated in FIG. 2 a differs from that illustrated in FIG.1 in that the space 75 in which the first piston surface 26 is situatedis not connected to the gas space 14 of the pressure accumulator 12 butis connected by way of two valves 71, 72 either to the pressure line 3or to the tank line 4 in a manner dependent on the position of thevalves. The gas pressure acts via a line 27 on a control surface 73 of apressure switching valve 71, wherein the pressure switching valveassumes a blocking position when the gas pressure reaches or exceeds therequired gas fill pressure. The required gas fill pressure, at which thevalve switches into the blocking position, is defined by the force of aspring 74, said force being directed counter to the gas force acting onthe control surface 73. The pressure switching valve is connected by wayof one port to the space 75 and by way of the other port to the tankline 4. If the valve is situated in the blocking position, asillustrated, the connection between the space 75 and the tank line isblocked, and oil cannot flow out of the space 75 to the tank 9. A checkvalve is arranged between the space 75 and the pressure line 3 such thatpressurized oil can flow from the pressure line via the check valve intothe space 75, that is to say onto the surface 26, but not in theopposite direction.

If, by activation of the pump 10, the percussive mechanism is suppliedwith pressurized oil via the pressure line 3, the percussive pistonperforms repeated working cycles and impacts against the tool 11. Oilflows out of the pressure line via the check valves 72 into the space 75and exerts on the piston 22 an oil force which opposes the spring force.The spring is configured such that the oil force exceeds the springforce and the piston is displaced to the left into the initial position,such that the spring is compressed and the bar connected to the pistonlikewise moves to the left, and the right-hand end of the bar no longerprotrudes beyond the housing surface 29 and is no longer visible. If thegas fill pressure in the gas space 14 corresponds to or is higher thanthe required gas fill pressure, the pressure switching valve assumes theblocking position and the oil cannot flow out of the space 75, that isto say the piston and the bar remain in the initial position, in whichthe bar is not visible. Only if the gas pressure undershoots therequired gas fill pressure is the pressure switching valve switched intothe pass-through position, allowing oil to flow out of the space 75 tothe tank via the pressure switching valve. This has the result that thepressure in the space 75 falls, and the spring force of the spring 24displaces the piston and the bar to the right, whereby the bar protrudesbeyond the housing surface 29 and appears, which signals to the operatorthat the gas fill pressure has been undershot. At the same time, withcorrespondingly large dimensioning of the lines and of the valves,pressurized oil can flow out of the pressure line via the check valveand the pressure switching valve to the tank, such that the pressure inthe pressure line falls to such an extent that the percussive mechanismcomes to a standstill and no longer imparts a percussive action. In thisway, operation with an excessively low gas fill pressure is prevented.The pressure switching valve may be equipped with a seat valve,preferably with an integrated check valve, such that in the blockingposition, the connection between the space 75 and the tank is shut offin a leakage-free manner. In this way, the indicator, that is to say thepiston 22 and the bar 23, would maintain their position for as long asthe gas pressure does not undershoot the target value, even over arelatively long period of time. The control surface 73 on the pressureswitching valve is sealed off by way of a seal or elastic diaphragm suchthat no gas can escape from the system via the control surface.

As an alternative to the embodiment as per FIG. 2 a, it is possible, asillustrated in FIG. 2 b, for the pressure switching valve 71 to beconnected to the pressure line 3 rather than to the tank line 4. Whenthe percussive mechanism is deactivated, which is necessary in order tobring the percussive piston into the lowermost position, which isrequired for the measurement of the gas pressure, the pressure line isunpressurized owing to the pump-side valve controller (not illustrated)or as a result of leakage at said controller, such that in this state,oil can flow out of the space 75 when the pressure switching valve hasbeen switched into the open position as a result of undershooting of therequired gas fill pressure. When oil flows out of the space 75 by thepressure switching valve, the spring 24 displaces the piston 22 and thebar 23 to the right, whereby a signal is output to indicate that the gasfill pressure has been undershot.

Furthermore, the check valve 72, which is arranged between the space 75and the pressure line, is integrated into the pressure switching valve71 such that, in the illustrated blocking position, when the percussivemechanism is deactivated and the pressure line is unpressurized, no oilcan flow out of the space 75 to the pressure line, but during theoperation of the percussive mechanism, when measurement of the gaspressure is not expedient, oil can flow out of the pressure line intothe space 75 in order to displace the indicator into its initialposition, in which the piston and bar assume their left-hand position.

The gas pressure acts on the piston of the indicator no longer directlybut indirectly, and controls the pressure switching valve, which definesthe end position of piston and bar. The piston and the bar no longerassume intermediate positions but can assume only two positions,specifically the initial position, in which the piston and bar aredisplaced to the left, and the signal position, in which the barprotrudes beyond the housing surface and is clearly visible, as a changein gas pressure to a level above a threshold value no longer directlyleads to a corresponding displacement of the piston and of the bar. Ifthe gas pressure exceeds the required gas fill pressure, the pressureswitching valve switches and leads to a considerable decrease inpressure in the space 75, triggering the displacement of the piston andof the bar from one position into the other position.

The percussive mechanism illustrated in FIG. 3 is equipped with a gaspressure indicator device 60 which differs from the embodimentillustrated in FIG. 1 in that the surface 26 of the piston 22 isconnected to the gas space 61 of the hydraulic accumulator 62. Thehydraulic accumulator can store pressurized oil of the hydraulic system.The gas space of the hydraulic accumulator is separated from an oilspace 64 by way of an elastic separating element 63. The oil space isconnected to the pressure line 3 directly or indirectly via a throttleor a valve and a line 65. For filling purposes or in order to releasethe gas charge, corresponding fittings such as for example hoses andpressurized gas bottles can be connected to the gas space 61 by way of avalve which is connected to the gas space. If an operating pressurehigher than the gas pressure within the gas space prevails in thepressure line, oil flows into the oil space and displaces the separatingelement in the direction of the gas space, whereby the volume of the oilspace is increased and that of the gas space is decreased. In this way,the gas is compressed up to a pressure which corresponds approximatelyto the pressure of the oil. Thus, when the oil demand of the percussivemechanism 1 is low, the oil that is delivered by the pump 10 can bestored in the hydraulic accumulator in order to reduce the pressureincrease in the pressure line, and when the demand of the percussivemechanism is high, oil can be released from said hydraulic accumulatorinto the pressure line in order to reduce the pressure drop in thepressure line. Intense pressure fluctuations and pressure peaks in thepressure line are thus prevented, performance is increased owing to themore constant operating pressure, and damage to components owing tointense pressure changes is prevented.

If the percussive mechanism has been deactivated and pressure built upby the pump 10 no longer prevails in the pressure line 3, it is also thecase that pressure no longer prevails in the oil space 64 of theaccumulator, and the gas pressure displaces the separating elementdownward, toward the oil space, until the gas space has assumed itsmaximum possible volume and the oil space has assumed its minimumpossible volume. In this state of the hydraulic accumulator, the gasfill pressure of the gas space can be measured. If an adequate gaspressure (target fill pressure) prevails in the gas space, the gaspressure effects a gas force acting on the surface 26 of the piston. Thesurface 26 and the spring 24 arranged on the opposite side of thepiston, which spring generates a spring force opposed to the gas force,are configured such that the gas force compresses the spring to such anextent that the piston assumes its right-hand end position, in which thepiston is fully retracted into the housing. If the gas pressure were tolie below the required target gas fill pressure, the spring force wouldexceed the gas force generated by the gas, and the piston would bedisplaced to the right, such that the bar is deployed out of the housingand appears clearly to the user, thus signaling to the user that therequired gas fill pressure has been undershot and replenishment of gasis necessary. Since the gas fill pressure is lower than the oil pressureprevailing in the pressure line during the operation of the percussivemechanism, it is the case that, during operation, oil flows into the oilspace, displaces the separating element and thus reduces the volume ofthe gas space, whereby the gas pressure increases to approximately thelevel of the oil pressure. In the presence of adequate gas fillpressure, the piston and the bar remain in their left-hand end positionsduring operation. The pressure indicator devices as per FIGS. 1, 2 a and2 b may, like that in FIG. 3, be used for monitoring the gas fillpressure of a hydraulic accumulator.

The gas pressure indicator device may be arranged directly on thehousing of the percussive mechanism or on a component connected to thehousing of the percussive mechanism, such as a valve block, or on theaccommodating housing surrounding the percussive mechanism.

The gas space or those components of the gas pressure indicator devicewhich are connected to the gas space of the accumulator may be equippedwith valves and connection means in order for a manometer or otherpressure indicator devices for determining the gas pressure toadditionally be connected, or in order for the gas pressure to bereduced or released through a discharge of the gas or for the gaspressure to be increased through a supply of gas into the gas space.Said valves and connections are not illustrated in the exemplaryembodiments but are known from known filling and testing devices forpressure accumulators of percussive mechanisms.

The bar may be equipped with several markings which denote different gaspressures.

Since, in the case of a constant volume, the gas pressure changes withchanging gas temperature, the bar may have provided on it multiplemarkings which denote the attainment of the target gas pressure atdifferent temperatures. In this way, it is possible for the gas pressureto be indicated even in the presence of gas temperatures which deviatefrom a predefined measurement temperature that is to be adhered to.

The position of the bar of the piston may be detected by way ofelectronics components for the purposes of triggering a signal, via anelectrical signal transmission means, to other locations, for example tothe carrier vehicle, or for the purposes of intervention into thehydraulic control of the percussive mechanism such that, in the event ofundershooting of the target gas pressure, the hydraulic supply to thepercussive mechanism is shut off or the operation of the percussivemechanism is stopped by intervention into the hydraulic control.

Contrary to the embodiments described above, in which, in the right-handend position of the piston, the bar is fully retracted into the housingand is not visible to the user, the indicator may also be designed suchthat, in said end position, a part of the bar protrudes beyond thehousing surface but said part is distinguished, by way of a marking, soas to clearly differ from that region of the bar which additionallyappears when the piston and the bar are moved to the right in thedirection of the other end position.

While specific embodiments of the invention have been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

What is claimed is:
 1. A device for monitoring a pressure within a gasspace, which is filled with a gas, preferably nitrogen, and/or isprestressed, of a pressure accumulator of a hydraulically drivenpercussive mechanism, the device comprising: a housing in which a pistonis mounted, together with a display element which extends through a facesurface of the housing, so as to be slidingly movable counter to a forceof a spring, wherein the piston divides the housing into a pressurechamber and a spring chamber, the spring acting on a spring chamber-sideworking surface, which is averted from the display element, of thepiston, and the pressure accumulator is at least indirectly operativelyconnected to the pressure chamber-side working surface, facing towardthe display element, of the piston, such that, in an event of a criticalpressure within the pressure accumulator being undershot, the piston andthe display element are displaced to such an extent that the displayelement protrudes out of the housing.
 2. A device according to claim 1,wherein the display element has at least one marking which, after theundershooting of the critical pressure and displacement of the displayelement, appears and signals the undershooting of the critical pressure.3. A device according to claim 2, wherein the display element hasaxially spaced-apart markings, wherein an appearance of said axiallyspaced-apart markings represents a measure for a value of the pressure.4. A device according to claim 1, wherein the display element is a barwhich is connected to the pressure chamber-side working surface of thepiston.
 5. A device according to claim 1, wherein the pressureaccumulator is connected directly, via a pressure line, to the pressurechamber-side working surface of the piston.
 6. A device according toclaim 5, wherein the pressure chamber of the pressure accumulator isconnected to a control surface of a spring-loaded pressure valve whichcan be transferred from a pass-through position into a blocking positioncounter to a force of a compression spring, wherein the pressure chamberof the housing is connected via a check valve to the pressure line ofthe hydraulics and, in the pass-through position, is relieved ofpressure to a tank.
 7. A device according to claim 5, wherein thepressure accumulator is connected to a control surface of aspring-loaded pressure valve which can be transferred from apass-through position into a one-way blocking position counter to aforce of a compression spring, wherein the pressure chamber of thehousing is, in the pass-through position, relieved of pressure to a tankand, in the one-way blocking position, connected via a check valve tothe pressure line of the hydraulics.
 8. A device according to claim 7,wherein the check valve is integrated in the spring-loaded pressurevalve.
 9. A device according to claim 7, wherein the check valve isarranged such that a flow of hydraulic fluid from the pressure line ofthe hydraulics via the check valve to the pressure chamber is possible,wherein a return flow from the pressure chamber is blocked via the checkvalve.
 10. A device according to claim 1, wherein said hydraulicallydriven percussive mechanism is one of a demolition hammer and a drillinghammer.
 11. A device for monitoring a pressure, comprising: ahydraulically driven percussive mechanism comprising a pressureaccumulator, said pressure accumulator comprising a fluid space, whichis filled with a fluid; a piston comprising a spring chamber-sideworking surface and a pressure chamber-side working surface; a displayelement, said spring chamber-side working surface facing in a directionaway from said display element, said pressure chamber-side workingsurface facing in a direction of said display element; a spring; ahousing, said piston being mounted in said housing, at least a portionof said display element being mounted in said housing, at least anotherportion of said display element extending through a face surface of thehousing, so as to be slidingly movable counter to a force of saidspring, wherein the piston divides the housing into a pressure chamberand a spring chamber, the spring engaging said spring chamber-sideworking surface, said pressure accumulator being at least indirectlyoperatively connected to the pressure chamber-side working surface ofsaid piston such that in an event of a critical pressure within thepressure accumulator being undershot, the piston and the display elementare displaced to such an extent that the display element protrudes outof the housing.
 12. A device according to claim 11, wherein the displayelement has at least one marking which, after the undershooting of thecritical pressure and displacement of the display element, appears andsignals the undershooting of the critical pressure.
 13. A deviceaccording to claim 12, wherein the display element has axiallyspaced-apart markings, wherein an appearance of said axiallyspaced-apart markings represents a measure for a value of the pressure.14. A device according to claim 11, wherein the display element is a barwhich is connected to the pressure chamber-side working surface of thepiston.
 15. A device according to claim 11, wherein the pressureaccumulator is connected directly, via a pressure line, to the pressurechamber-side working surface of the piston.
 16. A device according toclaim 15, wherein the pressure chamber of the pressure accumulator isconnected to a control surface of a spring-loaded pressure valve whichcan be transferred from a pass-through position into a blocking positioncounter to a force of a compression spring, wherein the pressure chamberof the housing is connected via a check valve to the pressure line ofthe hydraulics and, in the pass-through position, is relieved ofpressure to a tank.
 17. A device according to claim 15, wherein thepressure accumulator is connected to a control surface of aspring-loaded pressure valve which can be transferred from apass-through position into a one-way blocking position counter to aforce of a compression spring, wherein the pressure chamber of thehousing is, in the pass-through position, relieved of pressure to a tankand, in the one-way blocking position, connected via a check valve tothe pressure line of the hydraulics.
 18. A device according to claim 17,wherein the check valve is integrated in the spring-loaded pressurevalve.
 19. A device according to claim 17, wherein the check valve isarranged such that a flow of hydraulic fluid from the pressure line ofthe hydraulics via the check valve to the pressure chamber is possible,wherein a return flow from the pressure chamber is blocked via the checkvalve.
 20. A device according to claim 11, wherein said hydraulicallydriven percussive mechanism is one of a demolition hammer and a drillinghammer.